Production-scale formation of ultrahigh purity carbide powders

ABSTRACT

A method of forming ultrahigh purity carbide powders on a production scale which comprises blending metal oxide powders with elemental carbon powders and a partially polymerized furfuryl alcohol binder together with a suitable polymerization catalyst, extruding, polymerizing the binder, heating to 840* C for 48 hours in a soft vacuum, heating to 2,200* C for 1 hour in a helium atmosphere, cooling, crushing, and then heating to 2,000* C for 24 hours under a vacuum.

United States Patent Davidson et al.

[451 May 30, 1972 [54] PRODUCTION-SCALE FORMATION OF ULTRAHIGH PURITYCARBIDE POWDERS [72] Inventors: Keith Vernon Davidson; Donald H. Schell,

, both of Los Alamos, N. Mex.

[73] Assignee: The United States of America as represented by the UnitedStates Atomic Energy Commission [22] Filed: May 6,1970

[21] Appl.No.: 35,303

[52] US. Cl. ..264/0.5, 252/301 l R, 23/249 [51] Int. CL... ..G2lc 21/00[58] Field ofSearch ..264/0.5; 252/30l.1 R;

[56] References Cited UNITED STATES PATENTS 3,284,550 11/1966 Riley etal ..252/301.1

3,019,084 1/1962 Amsteim.

3,124,625 3/1964 Sheinberg 3,536,793 10/1970 Norman et a] 3,082,1633/1963 Ogard et al. ..264/0.5 3,375,073 3/1968 McMullen ..23/349 PrimaryExaminer-Benjamin R. Padgett Assistant Examiner-R. L. TateAttorney-Roland A. Anderson [57] ABSTRACT A method of forming ultrahighpurity carbide powders on a production scale which comprises blendingmetal oxide pow- 3 Claims, N0 Drawings PRODUCTION-SCALE FORMATION OFULTRAHIGl-l PURITY CARBIDE POWDERS The invention described herein wasmade in the course of, or under, a contract with the U. S. Atomic EnergyCommission. It relates to a method of forming carbides and, moreparticularly, to a method of forming carbide powders on a productionscale which are of the desired stoichiometry and free of vestigialsegregates and secondary phases.

Carbides of uranium and zirconium in solid solution having variousuranium contents are of interest as nuclear materials capable offunctioning at extremely high temperatures. However, the presence ofvestigial segregates such as free carbon and oxygen is undesirable inmost high-temperature applica tions. For example, when servicetemperatures exceed the carbide-carbon eutetic temperature, presence offree carbon may result in a structural failure because of formation of amelt. Subsolidus transformations, such as phase changes orprecipitations, also impose limitations on the practical applications ofthese carbides. Therefore, for reasons of compatibility, stoichiometricUC-ZrC solid solutions are essential, i.e., vestigial segregates such asfree carbon and secondary phases such as U C or UC cannot be tolerated.

In the prior art method of forming carbides, ultrahigh purity carbidessuch as those obtained utilizing the present invention could not be madeexcept in small quantities based on the procedure described in U. S.Pat. No. 3,284,550 by Riley and Davidson, issued on Nov. 8, 1966. Anessential step in the process of that patent is that the elementalconstituents of the carbides be are melted. As a consequence, themaximum amount of ultrahigh purity carbides that can be formed in asingle run according to the process therein disclosed is only about 75grams. This is essentially laboratory-scale production and henceextremely expensive if reasonably large quantities of the carbides aredesired.

An object of the present invention is therefore to provide a method ofpreparing ultrahigh purity carbides or solid solutions thereof inproduction quantities which are substantially free of secondary phasesand vestigial segregates such as interstitial carbon and oxygen.

Other objects of the invention will be apparent to one skilled in theart from reading the following specification.

The materials system utilized was (il um).

The desired ratio of uranium to zirconium was 1 mol uranium to 9 molszirconium. The starting materials were ZrO U 0.6 pm graphite flour,Thermax and Varcum binder. Experience has shown that this combination ofingredients produces a product which is very friable and easily groundinto a powder. A brief description of the essential ingredients follows:

ZrO The Zr0 was reactor grade oxide having a Fisher average particlesize of 5.4 m with a maximum particle size of 20 pm.

U0 The U0 was densified UO (0y or D-38) having an average particle sizeof about 4 pm.

0.6 p.11) graphite flour The stock used to make this flour was reactorgrade graphite which was ground and classified to a particle size ofless than 10 pm. The Fisher average particle size was 0.6 pm.

Thermax This material is essentially a spherical carbon black having afairly uniform particle size of about 0.3

Varcum The binder used was Varcum 8251 which is a partially polymerizedfurfuryl alcohol having a viscosity at room temperature of about 300cps. The Varcum was catalyzed with 4 g maleic anhydride per 100 cc ofresin.

Using the process of this invention, production quantities (i.e., about15,000 grams) of the solid-solution carbide can be produced in a verypure state in a single run. Typically, to produce 14,800 grams of the(UC) ,-(ZrC) the following amounts of ingredients are required:

0.6 pm graphite flour 1458 g Thermax 1458 g zit), 13,955 g Varcum 8253585 g The dry ingredients were first blended in a 16 qt twin-shellblender equipped with an intensifier bar and liquid addition apparatus.The ingredients were blended dry for 5 minutes using the intensifierbar. The Varcum binder was then added through the liquid additionapparatus over a period of about 15 minutes with the blender inoperation. The lids and walls of the blender were scraped of adheringmaterial and blending continued for 5 minutes.

The mix was then removed from the blender, passed through a meat choppertwice to warm and homogenize the mix, and extruded in a I00 ton press toa multihole configuration. The multiholed extrusions were extruded intographite fixtures at a rate of 20 ft/min.

The extrusions, held straight in the fixture, were heated to 7 7' 250 Cover a period of 63 hours in a circulating air oven to polymerize thetherrnosetting Varcum binder. They were then transferred to anotherfixture and furnace and heated in a soft vacuum to 840 C over a periodof 48 hours. They were then transferred again to another furnace andinductively heated in a susceptor to 2,200 C for 1 hour in a heliumatmosphere. After this treatment the extrusions were weak and easilybroken and crushed to a powder. Formulation was such that the hightemperature treated product should contain only (UC) -(ZrC) The freecarbon (0.6 pm and Thermax) added to the charge plus the carbon residuefrom the Varcum binder (estimated to be 45 percent of the binder) wassufficient to reduce the ZrO and U0 and convert them to the carbides.

The product made in this manner was single phase solid solution (UC)-(ZrC) however, it still contained 5,500 ppm free carbon and 6,100 ppmoxygen. Therefore, a final vacuum treatment at 2,000" C for 24 hours wasused to remove the excess carbon and lower the oxygen content. A typicalanalysis of carbides made in this manner follows:

Uranium 20.14 i 0.03 percent; zirconium 69.60 i 0.3

percent; total carbon 10.12 i 0.03 percent; free carbon 260 ppm; oxygen360 ppm.

X-ray diffraction analysis and metallographicexamination showed thisproduct to be single phase solid solution carbide. These results confirmthe feasibility of this procedure for the manufacture of largequantities of ultrahigh purity carbides.

Crushing of a hard brittle material such as (UC) -(ZrC) is very timeconsuming. It was found that the large batch of material preparedaccording to the process of this invention would crumble between onesfingers if fired in a helium atmosphere at 2,200 C for one hour. At2,350 C it would still break up fairly easily but showed evidence ofsintering and some of it had to be crushed in a carbide mortar andpestle. A temperature of 2,200 50 C is adequate for the necessarychemical reaction and also produces a desirable structure for reducingto powder.

What is claimed is:

1. A method of forming ultrahigh-purity uranium and zirconium carbidesand solid solutions thereof free of vestigial segregates and secondaryphases which comprises (a) mixing the elemental constituents with themetal or metals present in the form of oxide powders and the carbonpresent in the form of graphite and carbon black powder and a suitablebinder material and catalyst; (b) extruding; (c) polymerizing the bindermaterial; ((1) heating to 840 C in soft vacuum; (e) heating to 2,200 Cin helium; (f) crushing; and (g) heating to 2,000 C in vacuum.

2. The method of claim 1 wherein the binder material is partiallypolymerized furfuryl alcohol and the catalyst is maleic anhydride.

3. The method of claim 2 wherein step (d) is carried on for 48 hours,step (e) for one hour, and step (g) for 24 hours.

* l I l i

2. The method of claim 1 wherein the binder material is partiallypolymerized furfuryl alcohol and the catalyst is maleic anhydride. 3.The method of claim 2 wherein step (d) is carried on for 48 hours, step(e) for one hour, and step (g) for 24 hours.